In production processes, spectrometric measurements may be performed in gases, liquids, solids, and multiphase mixtures to obtain findings about the production process or a substance formed as a product of the process, in particular, its quantity or quality. From spectrometric measurements, values of measurands correlating to the concentration of educts and/or auxiliary materials of the process can be obtained. For example, in a biochemical production process, concentrations of nutrients and/or concentrations of metabolites of the microorganisms used in the production process and/or the concentration of the product produced in the process in a process medium may be monitored and the process controlled and/or regulated with the aid of the measurement data thus obtained. The process medium is generally contained in a process container, such as a reactor, a fermenter, or in a duct.
In absorption spectroscopy, a broadband light source is generally used, the light of which is directed through the medium to be investigated by using transparent windows or cuvettes, for example, and then analyzed in an optical spectrometer. The spectrometer thereby includes a detection unit comprising an aperture (for example, a slit), an imaging system and a detector, as well as the light source previously mentioned and the corresponding beam path.
The substances and mixtures of substances present in the medium may be identified by means of their characteristic absorption lines. Depending upon the molecule spectrum, different wavelengths are thereby of interest. Important here is that not only is the identification of individual lines relevant, but also their absolute signal strength, since information relating to the respective concentration may be calculated therefrom. In particular, many substances that are significant for applications of the process automation system have absorption lines in the ultraviolet (UV) spectral range. Therefore, for the analysis of such a mixture of substances, a UV absorption spectrometer requires, in particular, a detector designed for the wavelength range and an associated, appropriate light source.
Essentially, a distinction can be made between LED's, thermal lamps (e.g., halogen lamps), and gas discharge lamps.
Halogen lamps are ideal for spectrometric absorption measurements in a wide frequency range. They have a quite steady emission spectrum and emit continuous light. However, due to the practically temperatures of the lamp's filament, hardly any signal components are possible in the ultraviolet spectral range. Particularly in the shortwave light range below 350 nm, there is hardly any light intensity present. It is characteristic for gas discharge lamps that the relative light intensities through the resonances of the light sources do not form a continuous spectrum but, instead, sharply defined areas of high amplitude and areas with very low signal strength. This property is a key challenge for quantitative absorption measurements in particular, as it requires a detector with an extraordinarily high signal dynamic for a stable measurement.
A broadening of the sharp spectral lines, with gas discharge lamps, may, for example, be achieved in that the illuminating gas is brought under extremely high pressure and is operated at very high temperatures, as is the case, for example, with so-called xenon high-pressure lamps. The frequent nuclear shocks in the lighting medium caused by the high pressure result in a disruption of the quantum-mechanical transitions and a broadening of the sharp spectral lines. These types of high-pressure lamps often require an explosion-proof housing and separate cooling systems, which means that they are only employed in industrial sensors in exceptional cases. Therefore, it is necessary in practice to take appropriate steps to be able to perform absorption measurements in all frequency ranges despite the high signal dynamics, i.e., a lot of signal on the resonances and little signal in between.
In addition, in contrast to more or less continuously illuminating light sources for the visible spectral range, as are also used for ceiling lighting, considerably higher excitation energies are required for UV light sources. These may, for example, be achieved for gas discharge lamps that are controlled in individual flashes of high energy. These lamps can typically only be operated with very low repetition rates (e.g., 10 Hz) over long operating periods of several years.
As a final variant, UV LED's are also used as a light source. In contrast to the aforementioned lamps, however, these lamps have only an extraordinarily narrow frequency spectrum and for this reason are not suitable for a broadband analysis of the absorption characteristics for many wavelengths.
In summary, it can be noted that, for the majority of practical applications of industrial UV spectroscopy, only UV gas discharge lamps controlled with single flashes can be considered, and that, in practice, said lamps have in their emission spectrum a large number of sharp lines with strong intensity and, simultaneously, wavelength ranges in between with hardly any emitted light.
Today, cost-effective spectrometric detectors are usually constructed on a silicon basis and have, in particular, a characteristic wavelength-dependent sensitivity. This, in addition, increases the dynamic range necessary for broadband absorption in the detector.
In particular, CCD and CMOS detectors may be used for spectrometers. CMOS sensors have a high dynamic range that is actually desirable; however, they often do not have an optimal linearity property, which is particularly important for absorption measurements. With absorption measurements, the intensity of the measured transmitted spectrum is generally divided by the intensity of a reference measurement. That is, for example, a measurement for which the amount of light resulting when a sample liquid is filled into an irradiated liquid cuvette is divided by the intensities of a measurement for which pure water is poured into the transmission cuvette. Depending upon the absorption rate and wavelength, considerable differences in intensity can result in these two measurements. Particularly with high dynamic ranges, CMOS detectors often do not achieve any reliable quantitative results, particularly not if the temperature changes.
By contrast, CCD sensors usually have excellent linearity, but often have only a sharply reduced dynamic range. Usually, high-linearity CCD sensors are used, particularly for absolute transmission measurements. When continuously illuminating light sources are used, a very high dynamic range may be achieved as a result of the exposure time on the CCD detector being shortened or lengthened. However, this method is ruled out when UV flash lamps are used, since the typical flash pulse durations of 1 ns or less are considerably smaller than the minimum adjustable exposure times of CCD detectors, and an exposure with more than one flash pulse is not usually possible due to the only low flash pulse repetition rate possible.